At least four orders of fish including Salmoniformes (salmon and trout), Perciformes (perches, cichlids and damselfishes), Atheriniformes (silversides) and Tetraodontiformes (puffers and triggers) use the moon to time their reproduction – something known as lunar spawning synchronisation.

The moon goes through several phases over a 29 day period and fish use the different phases to allow them to time their spawnings. Although it occurs in some freshwater species, it is especially common in marine species, as the phase of the moon is directly linked to the tides, which can bring food and aid the dispersal of eggs and fry.

There are thought to be several reasons why fish adopt this strategy when spawning. For those species that provide brood care, such as cichlids, it’s easier for them to prevent predators eating their brood if they can see them. So they often lay their eggs so that the moon is bright when the eggs hatch, allowing them to see the brood and predators.

Many substrate spawning cichlids are believed to use this strategy, like Tilapia zillii pictured above, but at least one mouthbrooder, Cyprichromis leptosoma, has also been recorded using synchronised lunar spawnings.

Many fish species use the phase of the moon to allow them to know when to spawn and ensures that their eggs and sperm are ready at the right moment. Synchronised spawnings are also common in corals, which spawn en masse on specific nights.

Simply sticking a moonlight bulb on your tank won’t do much, but placing the tank somewhere near natural moonlight could have an effect.

This item was first published in the November 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.